1. Field of the Invention
The present invention relates to a solid-state image taking apparatus, a method for driving the solid-state image taking apparatus and an electronic apparatus employing the solid-state image taking apparatus.
2. Description of the Related Art
A Complementary Metal-Oxide Semiconductor, CMOS-type image sensor is a kind of solid-state image taking apparatus typically adopting the X-Y addressing system. The CMOS-type image sensor employs a pixel array section composed of unit pixels laid out two-dimensionally to form a matrix. In the following description, the unit pixel is also referred to simply as a pixel. Each of the unit pixels includes an opto-electric conversion device. The CMOS-type image sensor has a mechanism for sequentially scanning the pixel array section in pixel-row units and reading out signals generated by the pixels from the pixel array section. In an operation to sequentially scan the pixel array section in pixel-row units, an electronic shutter adopting a rolling shutter method is used. In this case, the length of an accumulation period of the pixel varies from pixel row to pixel row. Thus, in the operation to sequentially scan the pixel array section in pixel-row units, a phenomenon referred to as focal plane distortion occurs. In the focal plane distortion phenomenon, an image which is obtained as a result of an image taking operation carried out on an image taking object is distorted during the image taking operation.
In an application to take an image of an image taking object moving at a high velocity and not allowing a type of image distortion or in a sensing application requiring simultaneity of an image produced by an image taking operation, it is necessary to employ a global exposure mechanism (or a global shutter) for starting and ending an exposure operation with the same timings for all unit pixels in the pixel array section. The global exposure mechanism carries out an all-row simultaneous reset driving operation in order to reset unit pixels of the pixel array section for all pixel rows so that the accumulation operations of all unit pixels in the pixel array section can be started at the same time. Then, an all-row simultaneous electric-charge transfer operation is carried out in order to transfer electric charge to an electric-charge accumulation section such as a floating diffusion area at the same time so that the accumulation operations of all unit pixels in the pixel array section can be ended at the same time. Thus, it is possible to carry out a driving operation that results in simultaneity of accumulation periods of all individual unit pixels employed in the pixel array section. For details, see Japanese Patent Laid-Open No. 2006-311515 (Patent Document 1).
The opto-electric conversion device carries out an opto-electric conversion process to convert incident light into signal electric charge which is then transferred to the electric-charge accumulation section to be stored temporarily in the electric-charge accumulation section. By the way, in a row-after-row scan operation to read out the signal electric charge from the electric-charge accumulation section, the following problem is raised. Before the row-after-row scan operation to read out the signal electric charge from the electric-charge accumulation section, the signal electric charge stored in the electric-charge accumulation section deteriorates due to an aliasing signal. The aliasing signal includes a leak component of the electric charge and a noise component generated in an opto-electric conversion process carried out by the electric-charge accumulation section itself.
The leak component of the electric charge increases proportionally to the length of the elapsed time. In addition, the period ending at the operation to read out signal electric charge from the electric-charge accumulation section varies from row to row. If the driving operation is carried out to perform the operation to read out signal electric charge from the electric-charge accumulation section in a direction from the upper side of an image to the lower side of the image for example, the noise component increases, forming a shading shape in the direction from the upper side of an image to the lower side of the image. On top of that, while an image taking object serving as a noise source is undesirably moving in a period between the end of the electric-charge accumulation operation and the start of the electric-charge read operation, the noise component generated in an opto-electric conversion process carried out by the electric-charge accumulation section itself appears at a position different from the position of an image obtained as a result of the electric-charge accumulation operation. Thus, the image becomes unnatural.
In order to solve the problem described above, there has been proposed an image taking apparatus employing correction pixels separately from image taking pixels. The image taking pixel is a unit pixel used for accumulating and holding electric charge. On the other hand, the correction pixel is a pixel not used for holding accumulated electric charge but used for merely generating only an aliasing signal of an electric-charge holding section. For details, see Japanese Patent Laid-Open No. 2006-108889 (Patent Document 2). To put it concretely, an aliasing signal read out from a correction pixel in the vicinity of an image taking unit pixel is subtracted from a signal read out from the image-taking unit pixel as a signal including an aliasing signal in order to correct the aliasing signal read out from the image taking unit pixel.